Electro slag welding (ESW) is a conventional process. ESW creates a molten alloy pool by continuously passing an electrical current through one or more feed metal wires to a substrate surface through a molten and electrically conductive slag. There is no arc in the process, but electrical resistance in the slag and metal generates heat and continuously melts the feed metal, thereby adding to the molten metal pool. As the depth of the molten pool increases, the metal at the bottom of the pool is cooled and solidifies in a vertical direction, thereby adding freshly cast material to the substrate. This type of process is often used to join very thick plates such as for bridges or oil storage tanks. In those applications, the edges of the plates being joined contain two sides of the molten pool and water cooled copper shoes are used to contain the other two sides.
A disadvantage of electro slag welding is a lack of flexibility in the delivery of filler metal and electrical energy which are generally fixed by the initial hardware setup. This makes it difficult to optimize the heat distribution, to change the filler metal, or otherwise to modify the process once it is underway. As a result, ESW is not a process that is commonly used to repair an existing component.